1. Field of the Invention
The present invention is directed to a method for scanning an examination subject with a CT apparatus having a radiation source that can be displaced around a system axis and that emits a ray bundle that strikes a detector system that supplies scan data on the basis of which two-dimensional scan images of the examination subject are determined in an image calculating device.
2. Description of the Prior Art
CT devices are known that have a ray source, for example an X-ray tube, that directs a collimated, pyramidal ray bundle through the examination subject, for example a patient, onto a detector system composed of a number of detector elements. The ray source and—dependent on the structure of the Ct apparatus—the detector system as well are attached on a gantry that rotates around the examination subject. A support mechanism for the examination subject can be displaced along the system axis relative to the gantry. The position proceeding at which the ray bundle penetrates the examination subject and the angle at which the ray bundle penetrates the examination subject are constantly varied as a result of the rotation of the gantry. Each detector element of the detector system struck by the radiation produces a signal that represents a measure of the overall transparency of the examination subject for the radiation emanating from the ray source on its path to the detector system. The set of output signals of the detector elements of the detector system that is acquired for a specific position of the ray source is referred to as projection. A scan comprises a set of projections that were acquired at various positions of the gantry and/or various positions of the bearing mechanism. The Ct apparatus registers a number of projections during a scan in order to be able to construct a two-dimensional tomogram of a slice of the examination subject. A number of slices can be simultaneously registered with a detector system constructed of an array of a number of rows and columns of detector elements.
Large volumes of the examination subject are usually registered with sequence scanning or spiral scanning. In sequence scanning, the data are registered during the rotary motion of the gantry while the examination subject is in a fixed position and planar slices are thus scanned. Between the scanning of successive slices, the examination subject is moved into a new position wherein the next slice can be scanned. This procedure is continued until all slices determined before the examination have been scanned. In a spiral scan, the gantry with the radiation source rotates continuously around the examination subject while the patient table and the gantry are continuously displaced relative to one another along a system axis. With reference to the examination subject, the ray source thus describes a spiral path until the volume defined before the examination has been scanned. Images of individual slices are then calculated from the spiral data acquired in this way.
CT apparatuses are also known wherein the X-ray power can be modulated during the rotation of the ray source around the examination subject for scanning an examination subject having a non-circular cross-section. When, for example, a patient lying on his/her back is scanned, then the path of the X-rays through the body of the patient usually is longer in the horizontal direction than in the vertical direction. If a modulation of the X-ray power is not possible, then this must be set such that the signal quality supplied for the projection having the longest path of the radiation through the body still suffices for the calculation of proper images. The X-ray power for all other projections is thus set according to the attenuation profile that is dependent on the angular position of the radiation source. Such a method is disclosed, for example, in German OS 198 06 063.
CT apparatuses are particularly utilized in the medical field. In addition to examination for purely diagnostic purposes, interventions (for example, biopsies, centesis) are being increasingly implemented with CT monitoring. During the intervention, the position of medical instruments required for the implementation of the intervention, for example a needle, thus can be continuously checked. Given an activated ray source and manual guidance of such a medical instrument by an examining person, body parts of the examining person, for example a hand, that are located in the region between the focus and the detector system are permeated by the ray bundle and can be exposed to unattenuated radiation.
U.S. Pat. No. 5,873,826 discloses an X-ray CT apparatus wherein the radiation power of the X-radiator can be reduced at times during the scan for reducing the radiation dose supplied to an examining person. The volume region for which this reduction is effective is defined before the scan and is marked by means of a light source during the scan.